MEDIA COURTHOUSE -- An Upper Chichester baby sitter was convicted Friday on charges of third-degree murder and involuntary manslaughter for the Dec. 12, 2011, death of 2½-year-old David Miller Jr.

Heather Hess, 24, of the 1700 block of Hewes Avenue in Upper Chichester, was also convicted on counts of endangering the welfare of a child and aggravated assault after a jury of seven women and five men deliberated for about five and a half hours.

Hess had told the jury Thursday that she was playing "airplane" with Miller Jr. and her three children on the afternoon of Sunday, Dec. 11, swinging them around in a circle by their limbs when she fell and sent the young boy flying into a wall.

She said the boy fell asleep a few minutes later and she placed him on her bed, but later noticed his lips had turned blue and called paramedics at about 6:30 p.m. Miller Jr. was rushed to Crozer Chester Medical Center and later to the Children's Hospital of Philadelphia, where he died the following day.

Hess had initially told paramedics and police that the child had choked on a water bottle cap, a story she maintained for more than a year before testifying to the airplane accident Thursday.

The jury had already heard from multiple doctors and pathologists this week that it was impossible for Miller Jr. to have suffered the kind of major brain and spinal cord trauma that killed him from choking. Dr. Lucy Rorke-Adams, an expert in pediatric neuropathology at CHOP, said tearing of a major nerve bundle in the child's brain could only come from a rotational injury, likening the movement to "a bobble-headed doll."

"When there is tearing of the brain tissue, that is an indication that there was a rotational injury to the head, which is different than ... just falling straight down or back into an immovable flat object," said Dr. Aaron Rosen, the Philadelphia Assistant Medical Examiner who performed Miller's autopsy. "You wouldn't have tearing of the brain tissue (in a fall). That requires vectors or forces in different directions."

Rorke-Adams was also recalled Friday following Hess's testimony and said it was "impossible" for Miller Jr. to have sustained his injuries from hitting a wall, which would be a linear, rather than rotational, injury.

In closing statements, defense attorney Keith Garrity argued Deputy District Attorney Michael Galantino had failed to prove malice necessary for the murder charge or the level of reckless negligence needed to convict on involuntary manslaughter.

He urged the jury to think about the airplane story, noting Hess had checked the boy for injuries after the accident and found none, that she checked on him throughout the day after laying him on her bed and he appeared to be sleeping, and that she was in a panic upon finding him hours later with blue lips.

Garrity said Hess cared for the boy as one of her own and had watched him four or five times a week for months before the accident. It didn't make sense that she would suddenly snap, beating and shaking him violently enough to cause these injuries.

He also pointed to the child's already swollen head and body temperature 10 degrees below normal levels when he was admitted to Crozer Chester Medical Center, both of which more closely fit the timeframe of an injury occurring earlier in the day.

In his closing, Galantino hammered home that Hess had not told the truth from the moment she called 911 and reported a choking victim, a story she continued to sell to detectives even in the face of medical evidence disproving that explanation.

Galantino said Hess had never been concerned with saving Miller Jr., but with saving her own skin. He said her alleged violent outburst, immediate cover story and continued lies in the face of all medical evidence to the contrary constituted malice.

While Galantino said Hess had had a lot of time to think about the facts of the case and concoct a new story that she believed would fit that evidence, the fact remained that the child had suffered multiple blunt force trauma to the head and had been shaken so violently that his brain his spinal cord separated from his brain stem.

"This is a violent shaking, a raged shaking, not a shaking that somebody would do to revive a child," he said.